Problem: Luis is 3 times as old as Omar and is also 10 years older than Omar. How old is Luis?
Answer: We can use the given information to write down two equations that describe the ages of Luis and Omar. Let Luis's current age be $l$ and Omar's current age be $o$ $l = 3o$ $l = o + 10$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $l$ is to solve the second equation for $o$ and substitute that value into the first equation. Solving our second equation for $o$ , we get: $o = l - 10$ . Substituting this into our first equation, we get the equation: $l = 3$ $(l - 10)$ which combines the information about $l$ from both of our original equations. Simplifying the right side of this equation, we get: $l = 3l - 30$ Solving for $l$ , we get: $2 l = 30$ $l = 15$.